The present invention relates to a substrate for a liquid crystal display (LCD) device and a method for manufacturing the same.
Nowadays, LCD devices are the most widely used flat panel display devices. A LCD device comprises two substrate and liquid crystal layer injected between the two substrates. Each of the two substrates is provided with electrodes capable of generating electric field. Upon generating the electric field by the electrodes, liquid crystal molecules of the liquid crystal layer move and are rotated to a certain orientation, allowing light to pass through the liquid crystal layer with a certain refractive index. On the other hand, the two substrates are further coated with an alignment film on its inner surface, which can pre-tilt the liquid crystal molecules between the two substrates in a certain direction.
The alignment of liquid crystal molecules is very important for display quality of a LCD device, and the resultant alignment of liquid crystal molecules is affected by alignment characteristics such as the depth and the direction angle of the grooves formed in the alignment films. A conventional method for coating an alignment film is to coat alignment material by offset print method. In this method, a roller with one surface coated with alignment material is made to roll over and contact a surface of a substrate to be coated with alignment material, forming a layer of alignment film on the surface of the substrate, and then the alignment film is subject to a rubbing process to form grooves on the surface with certain alignment characteristics.
The substrate has different thicknesses in different regions, that is, steps are formed on the surface of the substrate, which cause the surface of the alignment film not to be planar enough. During forming grooves in the surface of the alignment film by rubbing, a high level portion is subject to a larger friction force, while a low level portion is subject to a smaller friction force. As a result, the part at the higher level is formed with deeper grooves, while the part at a lower level is formed with shallower grooves. The depths of the grooves affect the alignment controlling force of the alignment film, which force is for arranging the liquid crystal molecules in a certain direction. If the alignment controlling force is excessively large, a phenomenon that the liquid crystal molecules are prevented from moving occurs, resulting in a phenomenon of mura. On the other hand, if the alignment controlling force is too small, it cannot arrange the liquid crystal molecules in the certain direction, also resulting in the phenomena of mura, contrast ratio degradation and response readiness. Mura refers to a phenomenon that a part or entirety of an image is not uniform in tone when a LCD device displays the images. When the difference in alignment characteristics caused by a step is more severe, flicker and light leakage may occur. Thus the conventional LCD device embodies defects that greatly affect the display quality of the LCD device, such as non-uniformity in response speed and insufficiency in contrast ratio.